Battery Charge Time Calculator

Divide the amp-hours you need to add by the effective charge current: time (h) = Ah to add / (charge current in A x charging efficiency). For example, adding 30 Ah to a 50 Ah Li-ion battery (20% to 80%) with a 10 A charger at 95% efficiency takes 30 / (10 x 0.95) = 3.16 hours, about 3 hours 9 minutes. This calculator adds a taper factor for the slow final phase, so it is more honest than the bare formula.

It depends on capacity, charger current, and chemistry. A 12 V 100 Ah lead-acid battery charged from 20% to 100% (80 Ah to add) with a 10 A charger at 85% efficiency takes about 12.24 hours once you count the slow absorption phase above ~70% state of charge - the math is 50 Ah bulk / (10 x 0.85) plus 30 Ah taper x 1.8, not a straight 8-hour division. Enter your numbers above for an exact 12V battery charge time.

LiFePO4 charges efficiently and tapers late. A 280 Ah LiFePO4 battery charged from empty to full with a 40 A charger at 95% efficiency takes about 7.81 hours: 238 Ah of bulk charging at full rate plus 42 Ah of taper above 85% state of charge at 1.4x the time per unit. Use the battery tab above with the LiFePO4 preset for your own LiFePO4 charge time.

Charging efficiency (charge acceptance) is the fraction of charger energy that actually gets stored. Battery University data puts Li-ion coulombic efficiency above 99%, lead-acid around 90%, and NiMH/NiCd at roughly 70% on a slow charge and up to 90% fast. Practical charging-efficiency defaults in this tool: Li-ion and LiFePO4 95%, lead-acid 85%, NiCd 80%, NiMH 75%. Compare: charging 80 Ah into a 100 Ah battery at 10 A takes 8.42 hours for Li-ion but 10.35 hours for lead-acid - about 23% longer - because of lower efficiency plus a longer taper phase.

Charge time in peak sun hours = Wh to store / (panel watts x system derate x chemistry efficiency). Example: a 12 V 100 Ah lead-acid battery at 20% needs 960 Wh stored; a 200 W panel at 75% derate and 85% chemistry efficiency delivers 200 x 0.75 x 0.85 = 127.5 W effective, so 960 / 127.5 = 7.53 peak sun hours - about 1.67 days at 4.5 peak sun hours per day. Use the solar tab above for your panel and location to get an exact solar battery charge time.

Time = kWh to add / (charger kW x efficiency), plus a taper factor near full. A generic 60 kWh battery charged from 20% to 80% (36 kWh to add) takes 36 / (7.4 x 0.90) = 5.41 hours on a 7.4 kW home charger, or 36 / (50 x 0.88) = 0.82 hours (about 49 minutes) on a 50 kW DC fast charger, ignoring the heavy taper above 80% on DC. This generic EV charge time calculator covers any kWh-rated pack; if you want EV charging COST per 100 miles or trip cost with a 65-model vehicle database, use our EV charging cost calculator.

A 5000 mAh phone battery at 3.7 V stores 5000 x 3.7 / 1000 = 18.5 Wh. With a 10 W (5 V, 2 A) charger at 95% efficiency, that is 18.5 / (10 x 0.95) = 1.95 hours - about 2 hours. Real phones slow down past ~80% (the taper phase this tool models), so a full 0-100% charge usually reads a bit longer than the bare division suggests.

Chargers switch from constant-current (bulk) to constant-voltage or absorption mode near full: current tapers off to avoid overvoltage and heat. Li-ion does this above roughly 80% state of charge (about two-thirds of total charge time can go to topping charge, per Battery University), and lead-acid above roughly 70%, where the saturation phase makes a 'fast' lead-acid charge still take many hours. This tool models the taper with an adjustable multiplier instead of pretending charging is linear.

The taper factor multiplies the time for the slow final phase above the taper point. Defaults in this tool: Li-ion 1.5x above 80%, LiFePO4 1.4x above 85%, lead-acid 1.8x above 70% (long saturation), NiMH and NiCd 1.3x above 70%. Turn taper off for a best-case linear estimate, or adjust the multiplier to match your charger's curve. All taper values are labeled as planning defaults, not manufacturer specs.

First convert everything to watt-hours: Wh = mAh x V / 1000 (or Ah x V). A 20000 mAh power bank at 3.7 V holds 74 Wh. Then time = Wh / (charger watts x efficiency): with an 18 W USB-C input at 90% efficiency, 74 / (18 x 0.90) = 4.57 hours. Use the converter tab above for mAh/Ah/Wh swaps - for full unit conversions including coulombs and capacitors, see our electric charge converter.

Only up to the battery's charge-acceptance limit. Battery University recommends charging Li-ion at 0.5-1C (a 50 Ah battery accepts roughly 25-50 A); pushing past that causes heat and stress, not proportionally faster charging, and the taper phase ignores extra current anyway. Lead-acid is worse: it cannot be fast-charged at all, and the saturation phase takes hours regardless of charger size. Size the charger to the battery's C-rate, then let the calculator show the honest time.